The present invention relates to a stabilization apparatus in regard to interaction problems between turbine-generator shaft torsional oscillations and the dc transmission control system.
Dumping D.sub.m of the mechanical system of turbine-generators physically takes a positive value (D.sub.m &lt;0) although its value is small and is essentially stable. In the case of transmitting electric energy by the dc transmission, dumping D.sub.e of the electric system when viewed from the generator takes a negative value (D.sub.e &lt;0) by the influence of the constant current control considered as the fundamental control for the dc transmission in a low frequency range of 10 to 20 Hz ("HVDC-Turbine generator torsional interactions, A new design consideration" by M. P. Bahrman, E. V. Larsen et al. CIGRE SC14-04, 1980).
In the case where there exist some of natural frequencies of the mechanical shaft system of the turbine generator, which are included in such a low frequency range, when dc power becomes large, the resultant dumping (D.sub.m +D.sub.e) of the generator takes a negative value to become unstable. Thus, shaft torsional oscillations occur. For this problem, a stabilization scheme based on the dc control system is proposed.
Sub-synchronizing dumping control (which will be referred to as "SSDC" hereinafter) is effective for stabilization over an entire range of the shaft torsional frequency. When the turbine generator is expressed as N-mass model, there exist (N-1) natural frequencies. A circuit to collectively stabilize such oscillations is designed to apply the output of the stabilizing circuit to the output side of the current control regulator (which will be referred to as "ACR" hereinafter).
There are four problems to be considered in connection with interactions between turbine generator shaft torsional oscillations and the dc power-transmission system.
(1) It is considered that it is not simple to design a control system such that two closed loops for essential ACR and SSDC do not interfere with each other. For SSDC, an additional closed loop control is supplemented and its output is added to the output of ACR. Accordingly, it is considered that control design harmonized with the basic control is required for individual systems in order to obtain positive dumping (D.sub.e &gt;0) in the shaft torsional frequency range without damaging high response of the essential ACR.
(2) The know-how in regard to the detailed design of SSDC is so complicated that nobody understands other than developers. Such a detailed design includes a design to set dumping values over the entire range of the shaft torsional frequency to calculate frequency characteristic of SSDC for satisfying the set values to obtain high order transfer function to approximate to this, a design to obtain a transfer function of a feedback circuit for obtaining frequency characteristic of a desired open loop transfer function. Thus, the detailed design is complicated.
(3) The design of SSDC is dependent upon rather simple model in regard to the ac/dc system including the conventional basic control. Particularly in the case of the generator, since a simple voltage model on which only the angular velocity change .DELTA..omega. is taken into account is used, a simulator study or field verification is assumed to be required for the stabilization scheme based thereon, when applied to the actual system.
(4) The stabilization signal is obtained by deriving an internal voltage of the generator from the ac bus voltage by the compensation of the generator current in order to obtain a stabilization signal close to .DELTA..omega. of the generator as far as possible to input the internal voltage thus obtained to a frequency detector. However, it cannot be said that everything possible is done concerning the study as to whether or not any other signal can be used for the stabilization signal.